Power and free conveyor systems for moving bulky items through a manufacturing or assembly plant are well known. Such power and free conveyors include a "power" and a "free" conveyor track, generally disposed vertically with respect to each other. Operating within the power track is an endless drive chain with drive dogs periodically attached to the chain and extending toward the free track. These drive dogs are oriented to engage a trolley dog or actuator on a drive trolley operating within the free track. While the drive dogs are generally fixed in position relative to the drive chain, the trolley dogs on the drive trolleys are typically selectively retractable.
The free track generally follows the same path as the power track(s) but is spaced vertically relative thereto. As originally implemented, power and free conveyor systems were suspension systems with loads suspended from trolleys or carriers operating in the free track and with the power track disposed above the free track. These suspension systems have reached a high degree of sophistication and can include features such as the ability to stop and accumulate free trolleys in specific accumulating areas and transfer zones which include intersections where loads can be transferred between non-synchronous conveyor systems.
More recently, in response to the specific requirements of the automobile industry, floor mounted or "inverted" power and free systems have been developed. In these inverted systems, the power track and the free track are disposed beneath the floor of the factory, with the free track positioned above the power track. A plurality of load carriages are attached to the free trolleys through a slot in the factory floor. Each load carriage is usually attached to two or more load carrying trolleys with the load carriage being disposed above the floor and driven along the conveyor path by the associated load carrying trolleys.
These inverted systems have the capability of handling bulkier and heavier loads, such as automobile chassis, while minimizing many dangerous conditions found in suspension systems. For example, inverted systems allow workers to safely climb on and off of the load carriages and they eliminate the danger inherent in the swinging loads of suspension systems.
In such inverted power and free conveyors, the drive trolley is generally connected to the load carriage via a tow bar. The tow bar can be contained within the free track or it can be disposed above the surface of the factory floor. As a drive member or actuator on a drive trolley is engaged by a drive dog on the drive chain, the motion is abrupt, i.e. the drive trolley is jerked from a standing stop to the same speed as the drive chain in a split second. When the drive trolley is released, deceleration forces can be almost as strong. The drive trolleys are ruggedly designed so as to withstand these abrupt acceleration and deceleration forces. However, the acceleration and deceleration forces transmitted from the drive trolley to the load carrying trolleys via the tow bar can cause considerable stress and strain on the load carriages and the loads thereon.
It is well known to provide shock absorbing devices in power and free conveyor systems to dampen these abrupt acceleration and deceleration forces. While resilient bumpers and springs have been used, such devices, once compressed by a shock, tend to recover immediately, often setting up bouncing and vibration in the load. Thus, the more effective shock absorbing devices act to dampen the shock without setting up undesired bouncing or vibrations. Automotive type hydraulic or pneumatic shock absorbers have generally been the devices of choice. In such automotive-type shock absorbers, a piston operates within a damping chamber which includes a viscous fluid. The piston is connected to one end of the tow bar, for example, while the damping chamber is connected to the other end. Abrupt shocks are absorbed by forcing the viscous fluid through one or more orifices in the piston as it moves through the damping chamber.
Another shock absorbing mechanism, which is similar in concept to the automotive type shocks, uses a piston operating within a damping chamber which is partially filled with a particulate material, such as steel ball bearings or shot. Shocks are absorbed by the action of friction between the piston and the balls and between the balls themselves, as well as the effects of inertia on the balls. U.S. Pat. No. 5,027,715 to Archie S. Moore et al. (hereinafter the U.S. Pat. No. '715 patent), assigned to the present assignee, and which is herein incorporated by reference, teaches the use of such a particulate material shock absorbing device on a shock absorbing load carrier. In the U.S. Pat. No. '715 patent, the tow bar is eliminated by providing shock damping within the load carrier itself.
In power and free conveyor systems, the degree of damping required of a particular tow bar depends upon many factors, including the weight of the load carried, the speed of the drive chain in the power track, the track configuration, etc. In particulate shock absorbers such as the one taught by the U.S. Pat. No. '715 patent, the damping effect of the shock absorber is dependent upon many factors, including the relative diameters of the piston vs. the damping chamber, the size of the particulate material, i.e. the ball bearings, the relative coefficients of friction between the bearings and the piston and the chamber interior, and the number of bearings contained within the chamber, i.e. the degree to which the chamber is filled. Given a standard shock absorber configuration, of these variables, the one most readily changed is the percentage of fill of the chamber.
In shock absorbers such as that in the U.S. Pat. No. '715 patent, the size of the damping chamber is fixed, and thus the only way to change the percentage of fill is to disassemble the shock absorber and add or remove balls from the chamber. This is a complicated and labor intensive procedure which cannot readily be accomplished without taking the load carriage (or a tow bar incorporating such a shock absorber) out of service.
It is clear then, that a need exists for an improved design for a cushioned tow bar for a power and free conveyor transfer system which tow bar can be quickly and efficiently adjusted to provide selectively differing damping effects.